With reference to the article "Fiber Optic Switch for CATV Systems," 1990 NCTA Technical Papers, p. 339, an optical switch is a four port two state device which permits the connection of either input port to either output port. As shown in FIG. 1A, in the bar state, port 1 feeds port 2 and port 4 feeds port 3. In the cross state shown in FIG. 1B, port 1 feeds port 3 and port 4 feeds port 2. The transition between bar and cross state constitutes switching.
The implementation of optical switching in fiber video systems has been limited by significant insertion losses and signal degradation associated with such switches. These factors have limited the quality of signals which may be delivered using such systems. It is possible to perform switching by converting the optical signal to an electrical signal, performing the necessary switching, and converting the electrical signals back to optical signals. Typically, such signal switching is performed in a CATV system using RF A/B switches. However, such switches generally suffer from cross-talk and have a significant insertion loss, also limiting the ability to deliver high quality video signals.
The above-referenced article describes a commercially available optical switch which overcomes these deficiencies. Switching is achieved in this switch through the perturbation of a 2.times.2 fiber optic coupler. A three index tapered glass structure is made up of two fibers inserted into a tube of a third refractive index which is then necked down to a radius suitable to achieve a desired degree of coupling. For the switch application, a coupler is fabricated such that 100 percent of the light is coupled from the input fiber to the second fiber. With bending, the percent of light coupled to the second fiber can be varied from 100 percent to 0 percent. This arrangement provides a switch with low optical losses, low back reflection, and high reliability.
The switch may be switched either manually via a switch, for example, or electronically. The switching is accomplished by solenoids which are physically mounted such that one solenoid attracts a permanent magnet in the coupler carriage, while the other solenoid opposes the polarity of the permanent magnet mounted on the other side of the coupler carriage. This produces a lateral translation of the coupler carriage and bends the coupler, switching the light input to the other port. The carriage remains latched in this position until switched again. A switch state signal indicates which state (cross or bar) the switch is in.